1. Field of the Invention
The present invention relates to hearing aids. The invention more specifically relates to a method of sound processing in a hearing aid. The invention also relates to a hearing aid adapted to carry out such sound processing.
In the context of the present disclosure, a hearing aid should be understood as a small, microelectronic device designed to be worn behind or in a human ear of a hearing-impaired user. A hearing aid system may be monaural and comprise only one hearing aid or be binaural and comprise two hearing aids. Prior to use, the hearing aid is adjusted by a hearing aid fitter according to a prescription. The prescription is based on a hearing test, resulting in a so-called audiogram, of the performance of the hearing-impaired user's unaided hearing. The prescription is developed to reach a setting where the hearing aid will alleviate a hearing loss by amplifying sound at frequencies in those parts of the audible frequency range where the user suffers a hearing deficit. A hearing aid comprises one or more input transducers, typically microphones, a microelectronic circuit comprising a signal processor, and an acoustic output transducer, also referred to as a receiver or a speaker. The signal processor is preferably a digital signal processor. The hearing aid is enclosed in a casing suitable for fitting behind or in a human ear.
The mechanical design has developed into a number of general categories. As the name suggests, Behind-The-Ear (BTE) hearing aids are worn behind the ear. To be more precise, an electronics unit comprising a housing containing the major electronics parts thereof is worn behind the ear. An earpiece for emitting sound to the hearing aid user is worn in the ear, e.g. in the concha or the ear canal. In a traditional BTE hearing aid, a sound tube is used to convey sound from the output transducer, which in hearing aid terminology is normally referred to as the receiver, located in the housing of the electronics unit, and to the ear canal. In some modern types of hearing aids a conducting member comprising electrical conductors conveys an electric signal from the housing and to a receiver placed in the earpiece in the ear. Such hearing aids are commonly referred to as Receiver-In-The-Ear (RITE) hearing aids. In a specific type of RITE hearing aids the receiver is placed inside the ear canal. This category is sometimes referred to as Receiver-In-Canal (RIC) hearing aids.
In-The-Ear (ITE) hearing aids are designed for arrangement in the ear, normally in the funnel-shaped outer part of the ear canal. In a specific type of ITE hearing aids the hearing aid is placed substantially inside the ear canal. This category is sometimes referred to as Completely-In-Canal (CIC) hearing aids. This type of hearing aid requires an especially compact design in order to allow it to be arranged in the ear canal, while accommodating the components necessary for operation of the hearing aid.
Hearing loss of a hearing impaired person is quite often frequency-dependent. This means that the hearing loss of the person varies depending on the frequency. Therefore, when compensating for hearing losses, it can be advantageous to utilize frequency-dependent amplification. Hearing aids therefore often provide to split an input sound signal received by an input transducer of the hearing aid, into various frequency intervals, also called frequency bands, which are independently processed. In this way it is possible to adjust the input sound signal of each frequency band individually to account for the hearing loss in respective frequency bands. The frequency dependent adjustment is normally done by implementing a band split filter and compressors for each of the frequency bands, so-called band split compressors, which may be summarised to a multi-band compressor. In this way it is possible to adjust the gain individually in each frequency band depending on the hearing loss as well as the input level of the input sound signal in a respective frequency band. For example, a band split compressor may provide a higher gain for a soft sound than for a loud sound in its frequency band.
The filter banks used in such multi-band compressors are well known within the art of hearing aids, but are nevertheless based on a number of tradeoffs. Most of these tradeoffs deal with the frequency resolution as will be further described below.
There are some very clear advantages of having a high resolution filter bank. The higher the frequency resolution, the better individual periodic components can be distinguished from each other. This gives a much finer signal analysis and enables more advanced signal processing such as noise reduction or feedback canceling.
The reasons for wanting a low resolution filter bank are more subtle. One aspect relates to the temporal smearing in the filter bank. Temporal smearing is the result of a wideband signal exciting several bands in the filter bank, since the time delay of the frequency band varies and therefore the output is temporally smeared when the frequency bands are summed together.
2. The Prior Art
In state of the art hearing aids it is well known to apply the hearing gain based on the frequency and the input level.
It is also well known within the art, to adapt the hearing aid signal processing based on a detection of whether speech is present in the signal. In more advanced systems the signal processing may even be based on whether the speech is voiced or unvoiced.
US-A1-20120008791 discloses a hearing aid with two-stage frequency transformation. Some of the processing, for example the amplification, is carried out after high stopband attenuation in the first stage. An increased frequency resolution is achieved in a second stage before the back-transformation in the first stage, which is favorable for noise reduction, for example.
EP-A1-2383732 discloses a hearing aid including a speech analysis unit, which detects a consonant segment and a vowel segment within a detected sound segment, and a signal processing unit which temporally increments the consonant segment detected by the speech analysis unit and temporally decrements at least one of the vowel segment and the segment acoustically regarded as soundless detected by the speech analysis unit.
It is a feature of the present invention to provide a method of sound processing in a hearing aid that provides improved frequency filtering.
It is another feature of the present invention to provide a method of sound processing in a hearing aid that provides improved speech intelligibility based on a detection of whether voiced or unvoiced speech is present.
It is still another feature of the present invention to provide a method of sound processing in a hearing aid that provides improved frequency transposition in a hearing aid.
It is yet another feature of the present invention to provide a method of sound processing in a hearing aid that provides improved means for estimation of frequencies in a hearing aid signal.